I have never disassembled a stepper, but have overhauled servo's in many cases.
The theory is that a 'Keeper' should be used, either around a rotor, or in a stator to retain the magnetic field, otherwise the P.M. magnetic field can diminish.
So far with servo's I have never had a problem in not using a 'Keeper', many motor manufacturers will state there is no detriment to removing the rotor, due to the quality of the rare earth magnets used.
As an aside, I have many stator magnets removed from servo motor that were unrepairable, using them for various functions around the shop, after many years, I cannot detect any weakening of these very strong magnets.
Al.
CNC, Mechatronics Integration and Custom Machine Design
“Logic will get you from A to B. Imagination will take you everywhere.”
Albert E.